This disclosure relates to ram air turbines utilized to provide electric and hydraulic power to an aircraft. More particularly, this disclosure relates to a housing of a ram air turbine generator.
A ram air turbine is used to generate supplemental power in an aircraft by extracting power from an air stream along the exterior of the aircraft during flight. The ram air turbine includes a turbine that drives an electric motor and/or hydraulic pump. In operation, the turbine is moved from a stowed position within the aircraft to a deployed position just outside of the aircraft such that the blades of the turbine are in the air stream and also have an operating clearance with the aircraft. The turbine is mounted at the end of a strut and drives a turbine drive shaft that in turn drives the electric motor and/or hydraulic pump.
The ram air turbine may experience extreme loads, such as during high level, short duration events (HLSDs). During an aircraft engine blade loss event, the severe HLSD vibrations occur first as the engine spools down. Then, as it continues to turn due to air loads, a high unbalance load continues to drive the longer duration windmilling vibrations. Either or both of these vibrations could significantly reduce the fatigue life of RAT components.
As known, windmilling is generally unpowered aircraft engine rotation that occurs at frequencies below most RAT resonant frequencies. However, HLSD is high level resonant vibration wherein one or more RAT modes are excited to resonance as the engine spools down and this causes high loads through the RAT housings. As the aircraft engine speed drops, the excitation frequency seen by the RAT sweeps from high to low frequencies, passing through normal RAT resonance frequencies on the way down. Thus it is not generally possible to design a RAT with natural frequencies that avoid a HLSD sweep event. Strengthen the RAT helps the RAT to endure the event. Also, the duration of spool down time is a fraction of the windmilling duration and so the number of fatigue cycles for HLSD is relatively small. Consequently, a practical design of the load bearing housings is feasible with appropriate load paths and careful attention to minimizing stress concentrations
A generator housing of the ram air turbine needs to withstand these loads while accommodating changes in part dimensions due to thermal variations.